1. Field of the Invention
The present invention relates to communications-related systems and methods as well as digital signal processing devices and methods. More particularly, the invention is directed to the field of spread spectrum (SS) communication that employs a SS transmitter and a SS receiver, or SS transceiver configured to convey a message in a transmitted SS signal by spectrally spreading the message signal on transmission and correlating on reception so as to “despread” the SS signal and recover the message signal.
2. Description of the Background
Conventional narrowband (i.e., non-spread spectrum) radio communication devices transmit signals in frequency bandwidths that are roughly equivalent to a message signal bandwidth (or information bandwidth). These devices typically use a radio-frequency (RF) carrier derived from a frequency reference (i.e., a device that produces a precise frequency, although the accuracy of the frequency usually depends on the cost of the device) and modulate the message onto an RF carrier. Common conventional message modulation methods such as amplitude modulation (AM), phase modulation (PM) or frequency modulation (FM) cause the RF carrier to occupy more bandwidth than the RF carrier alone, but the total bandwidth for the modulated RF carrier is relatively narrow. As such, interfering signals (e.g. jammers) that are transmitted in the same bandwidth as the modulated RF carrier can effectively “jam” the desired signal and prevent a receiver from reproducing the message signal. Aside from jamming, disturbances in the communications path between the transmitter and receiver can interfere with reception.
Spread spectrum radio communication addresses the shortcomings of narrowband radio communications by combining a wideband spreading signal with the message signal so as to spectrally spread the message signal. In these types of systems. the transmitter also modulates an RF carrier with a message, as with the narrowband systems, but then adds one more step by modulating the resulting signal with a wideband, noise-like spreading signal (e.g. a PN code). Consequently, the message signal is spread in frequency over a much larger bandwidth, typically by ten to a thousand fold. Common spread spectrum techniques include frequency hopping and direct sequence. Frequency hopping systems drive (i.e., “hop”) the message modulated carrier to frequencies following a pseudo-random pattern defined by a spreading code. Direct sequence systems combine a spreading code with the message modulated carrier to create a signal which occupies about the bandwidth of the spreading signal.
Narrowband interference signals transmitted at the same frequency as a portion of the desired spread signal, “jam” the spread signal by an amount proportional to the ratio of jammer bandwidth to spread bandwidth. At most, the narrowband interfering signal is attenuated by a “process gain” of the spread spectrum system, where process gain is defined as a ratio of spread signal bandwidth to message signal bandwidth. For similar reasons, spread spectrum signals also offer some degree of immunity to multipath fading and distortions. However, in applications where a SS receiver is attempting to receive a distant SS transmitter, the process gain of the system may be insufficient to overcome the undesirable effects of a nearby narrowband jammer.
A narrow band-reject filter placed prior to the spread spectrum correlator provides narrowband interference rejection far in excess of the process gain. Applications are enabled where a distant SS transmitter is received by a SS receiver in the presence of a nearby narrowband jammer. Many techniques have been disclosed which practice the narrow band-reject filter. Some are accomplished in the frequency domain with digital signal processing (DSP). These techniques as taught require extensive computing resources and are therefor relatively expensive to implement.
In particular, J. D. Laster and J. H. Reed, (“Interference Rejection in Digital Wireless Communications,” IEEE Signal Processing Magazine, May 1997) serves as a bibliography of interference rejection techniques that have been published in recent years. The techniques cover both spread spectrum and narrowband methods.
Souissi (U.S. Pat. No. 5,671,247) teaches a frequency domain technique to remove narrowband jammers from a received spread spectrum signal. Souissi converts a received signal into the frequency domain where the signal components are represented by magnitude and phase. The magnitude of all signal components are normalized, thereby reducing the effects of narrowband jammers. The resultant signal components are then converted back into the time domain for message demodulation.
Blanchard (U.S. Pat. No. 5,612,978) also teaches the use of frequency domain techniques to reject narrowband interference. Frequency bins in which narrowband energy is detected are removed. The circuit contains a delay element to account for the time required for the FFT processing. It also requires the use of noise estimation for proper operation.
The use of a prime factor FFT along with time-frequency correlation for rapid and computationally efficient spread spectrum synchronization is disclosed in patent application Ser. No. 08/929,891 and is in its entirety incorporated by reference herein.
The implementation of filter banks using a Fourier transform along with a data taper window to control spectral leakage is disclosed in “Window Choices Become Crucial in High-dynamic-Range FFT Processing” by Charles Gumas, May, 1997, Personal Engineering.
A general digital signal processing reference is “Handbook for Digital Signal Processing” by Sanjit K. Mitra and James F. Kaiser, 1993, John Wiley & Sons, Inc.